The present invention relates to wireless communications, and more particularly, to code division multiple access (CDMA) communications.
In conventional CDMA systems, user signals are typically separated using user-specific spreading codes. In IS-95 and IS-2000 systems, for example, forward link (base station to mobile terminal) Traffic Channels are separated using Walsh codes assigned to users during call setup. Reverse link (mobile terminal to base station) traffic channels are typically separated using what is referred to as a “long code,” which is a very long pseudo-random code (e.g., 242−1 chips).
The reverse link traffic channel code is typically generated in two steps. In a first step, a pseudo-random code that is the same for all users is generated. In a second, “code masking” step, the common pseudo-random code is processed using a mask specific to the user. In typical IS-95 and IS-2000 systems, the mask used is a function of the Electronic Serial Number (ESN) of the terminal.
In order to recover a reverse traffic channel transmitted by a terminal, a base station typically synchronizes itself with the reverse link signal in a process commonly referred to as code acquisition. In a typical code acquisition process, a base station searches through the reverse link traffic channel code until it can gather sufficient information to synchronize with the terminal. Because of the length of the typical reverse link traffic channel code, it may not be practical to search the entire length of the code. Accordingly, the IS-95 and IS-2000 standards require that terminals transmit on the reverse link in synchronism with the System Time reference. This can allow the base station to search a relatively small amount-of code to acquire the reverse link traffic channel code, typically an amount of the code corresponding to two-way propagation delay and multipath delay, e.g., on the order of plus and minus a few chips.
Under current IS-95 and IS-2000 standards, no two mobile terminals are allowed to have the same ESN. If mobile terminals having the same reverse link traffic channel code are operating in close proximity, e.g., transmitting to the same base station, their reverse link traffic channels may coincide, leading to crosstalk. The ESN currently used is a 32-bit value, thus allowing for billions of unique ESNs. However, it has been predicted that ESN values will be exhausted by 2004. Moreover, because there is an ongoing trend to provide for worldwide compatibility among wireless devices and systems, and because ESNs are typically assigned by various entities in various countries, enforcing “no duplicate ESN” rules may become problematic.